Depleted silicon-on-insulator capacitive MOSFET for analog microcircuits

ABSTRACT

Dual gate FD-SOI transistors are used as MOSFET capacitors to replace passive well capacitors in analog microcircuits. Use of the dual gate FD-SOI devices helps to reduce unstable oscillations and improve circuit performance. A thick buried oxide layer within the substrate of an FD-SOI transistor forms a capacitive dielectric that can sustain high operating voltages in the range of 1.2 V-3.3 V, above the transistor threshold voltage. A secondary gate in the FD-SOI transistor is used to create a channel from the back side so that even when the bias voltage on the first gate is small, the effective capacitance remains higher. The capacitance of the buried oxide layer is further utilized as a decoupling capacitor between supply and ground. In one example, a dual gate PMOS FD-SOI transistor is coupled to an operational amplifier and a high voltage output driver to produce a precision-controlled voltage reference generator. In another example, two dual gate PMOS and one dual gate NMOS FD-SOI transistor are coupled to a charge pump, a phase frequency detector, and a current-controlled oscillator to produce a high-performance phase locked loop circuit in which the decoupling capacitor footprint is smaller, in comparison to the conventional usage of passive well capacitance.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is a divisional of U.S. patentapplication Ser. No. 14/219,786, filed Mar. 19, 2014, which is herebyincorporated by reference in its entirety.

BACKGROUND

Technical Field

The present disclosure generally relates to the use of fully depletedsilicon-on-insulator (FD-SOI) transistors as substitutes for passivecapacitors, and to applications of such capacitive transistor structuresin improving performance of analog microcircuits.

Description of the Related Art

Analog microcircuits and, in particular, integrated analog circuitsbuilt on semiconductor substrates, are used widely in medicalelectronics, aerospace systems, and devices equipped with RFcommunications capability. Such RF-equipped devices include smartphones, tablet computers, microelectronic sensors, and the like.Specific examples of analog microcircuits include reference voltagegenerator circuits and phase locked loops. Reference voltage generatorcircuits are used to produce a constant reference voltage involtage-regulated devices such as power supplies and analog-to-digitalconverters. Phase locked loop (PLL) circuits are used to perform phasematching between input and output signals of oscillators. Such circuitscan be used, for example, in microprocessors for clock synchronization,and in telecommunications for signal demodulation and frequencysynthesis.

Capacitors are frequently used as components in analog microcircuits.Various structures that are present on integrated circuit chips can bemade available to circuit designers for use as parallel platecapacitors, or such capacitors can be specially designed. Alternatively,it is possible for metal-oxide-semiconductor field effect transistors(MOSFETs), which are ubiquitous on integrated circuit chips, to be usedas capacitors in various microcircuits if the components are connectedin a particular manner. In a field effect transistor (FET), a thin gateoxide layer between the gate and the channel can be considered acapacitive dielectric, and the gate and the channel regions can beconsidered two parallel plates of a capacitor. However, because typicalgate oxide layers are only a few nm thick, the gate oxide layer tends tobe vulnerable and can break down easily in response to application of agate voltage that is slightly too large. Furthermore, transistors areactive, three-terminal devices whereas capacitors are passive,two-terminal devices that are generally much simpler and less expensiveto manufacture. For at least these reasons, it is generally notcustomary to connect transistors in such a way that they can be used ascapacitors.

BRIEF SUMMARY

Dual gate FD-SOI transistors are used as MOSFET capacitors in analogmicrocircuits to reduce unstable oscillations and to improve circuitperformance. A primary gate spaced apart from the channel by a gatedielectric layer controls current in the channel of the device. Asecondary gate also controls current in the channel, the secondary gatebeing spaced apart from the channel by a thicker buried dielectric layerwithin the substrate of the SOI transistors. The thick buried oxidelayer acts as a capacitive dielectric that can sustain high operatingvoltages in the range of 1.2 V-3.3 V, above the transistor thresholdvoltage, which is typically about 0.5 V. In one embodiment, thedielectric of the gate and the buried layer are oxides, such as SiO₂.Such FD-SOI devices can be substituted for parallel plate capacitorcomponents in integrated circuits, to improve circuit performance andefficiency.

In one exemplary embodiment, a dual gate PMOS FD-SOI transistor iscoupled to an operational amplifier and a high voltage output driver toproduce a precision-controlled voltage reference generator.

In another exemplary embodiment, two dual gate PMOS transistors and onedual gate NMOS FD-SOI transistor are coupled to a charge pump, a phasefrequency detector, and a current-controlled oscillator to produce ahigh-performance phase locked loop circuit in which the decouplingcapacitor footprint is significantly smaller.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements.The sizes and relative positions of elements in the drawings are notnecessarily drawn to scale.

FIGS. 1A and 1B are cross sections of existing silicon-on-insulatordevices that illustrate partially depleted and fully depleted channels.

FIG. 2 is a cross section of one type of prior art dual gate SOItransistor.

FIG. 3 is a schematic diagram of an existing reference voltage generatorcircuit that includes a passive well capacitance C1.

FIG. 4 is a plot of effective capacitance as a function of appliedvoltage across an NWELL capacitor.

FIG. 5 is a schematic diagram of a reference voltage generator circuitthat includes a dual gate FD-SOI MOSFET device, as described herein.

FIG. 6A is a cross-section of a dual gate PMOS FD-SOI MOSFET deviceconfiguration, as described herein.

FIG. 6B is a cross-section of a dual gate flip well PMOS FD-SOI MOSFETdevice configuration, as described herein.

FIG. 6C is a schematic diagram of the dual gate PMOS FD-SOI MOSFETdevice configurations shown in FIGS. 6A and 6B.

FIG. 7 is a flow diagram of steps in a process of fabricating the dualgate PMOS FD-SOI MOSFET shown in FIG. 6A.

FIG. 8A is a schematic diagram of a conventional PWELL capacitoroperated at a supply voltage of 3.3 V.

FIG. 8B is a schematic diagram of a dual gate PMOS transistor for use asa capacitor in a circuit having a supply voltage of 3.3 V.

FIG. 8C is a schematic diagram of an FD-SOI dual gate PMOS transistorfor use as a capacitor in a circuit having a supply voltage of 3.3 V, asdescribed herein.

FIG. 8D is a plot of simulated capacitance vs voltage (C-V) curves forthe device configurations shown in FIGS. 8A-8C.

FIG. 9A is a schematic diagram of a prior art charge pump phase lockedloop circuit that includes three capacitors.

FIG. 9B is a schematic diagram of a charge pump phase locked loopcircuit that includes three capacitive dual gate FD-SOI MOSFET devices,as described herein.

FIG. 10A is a cross-section of a dual gate NMOS FD-SOI MOSFET deviceconfiguration, as described herein.

FIG. 10B is a cross-section of a dual gate flip well NMOS FD-SOI MOSFETdevice configuration, as described herein.

FIG. 10C is a schematic diagram of the dual gate NMOS FD-SOI MOSFETdevice configurations shown in FIGS. 10A and 10B.

FIG. 11A is a schematic diagram of a conventional NWELL capacitor towhich a bias voltage is applied.

FIG. 11B is a schematic diagram of a dual gate NMOS transistor for useas a capacitor in a circuit having an applied bias voltage.

FIG. 11C is a schematic diagram of an FD-SOI dual gate NMOS transistorfor use as a capacitor in a circuit having a supply voltage of 3.3 V, asdescribed herein.

FIG. 11D is a schematic diagram of a flip-well FD-SOI dual gate NMOStransistor CNMOSFW, for use as a capacitor in a circuit having a supplyvoltage of 3.3 V, as described herein.

FIG. 11E is a plot of simulated C-V curves for the device configurationsshown in FIGS. 11A-11D.

FIG. 12A is a schematic diagram of a conventional NWELL capacitoroperated at a supply voltage of 1.2 V.

FIG. 12B is a schematic diagram of a dual gate NMOS transistor for useas a capacitor in a circuit having a supply voltage of 1.2 V.

FIG. 12C is a schematic diagram of an FD-SOI dual gate NMOS transistorfor use as a capacitor in a circuit having a supply voltage of 1.2 V, asdescribed herein.

FIG. 12D is a schematic diagram of a flip-well FD-SOI dual gate NMOStransistor CNMOSFW, for use as a capacitor in a circuit having a supplyvoltage of 1.2 V, as described herein.

FIG. 12E is a plot of simulated C-V curves for the device configurationsshown in FIGS. 12A-12D.

FIG. 13A is a schematic diagram of a conventional PWELL capacitoroperated with a bias voltage.

FIG. 13B is a schematic diagram of a dual gate PMOS transistor for useas a capacitor in a circuit having an applied bias voltage.

FIG. 13C is a schematic diagram of an FD-SOI dual gate PMOS transistorfor use as a capacitor in a circuit having a supply voltage of 1.2 V, asdescribed herein.

FIG. 13D is a schematic diagram of a flip-well FD-SOI dual gate PMOStransistor CNMOSFW, for use as a capacitor in a circuit having a supplyvoltage of 1.2 V, as described herein.

FIG. 13E is a plot of simulated C-V curves for the device configurationsshown in FIGS. 13A-13D.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various aspects of thedisclosed subject matter. However, the disclosed subject matter may bepracticed without these specific details. In some instances, well-knownstructures and methods of semiconductor processing comprisingembodiments of the subject matter disclosed herein have not beendescribed in detail to avoid obscuring the descriptions of other aspectsof the present disclosure.

Throughout the specification and claims that follow, the word “comprise”and variations thereof, such as “comprises” and “comprising” are to beconstrued in an open, inclusive sense, that is, as “including, but notlimited to.”

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases in “oneembodiment” or “in an embodiment” in various places throughout thespecification are not necessarily all referring to the same aspect.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more aspects of the presentdisclosure.

Reference throughout the specification to integrated circuits isgenerally intended to include integrated circuit components built onsemiconducting substrates, whether or not the components are coupledtogether into a circuit or able to be interconnected. Throughout thespecification, the term “layer” is used in its broadest sense to includea thin film, a cap, or the like.

Reference throughout the specification to conventional thin filmdeposition techniques for depositing silicon nitride, silicon dioxide,metals, or similar materials include such processes as chemical vapordeposition (CVD), low-pressure chemical vapor deposition (LPCVD), metalorganic chemical vapor deposition (MOCVD), plasma-enhanced chemicalvapor deposition (PECVD), plasma vapor deposition (PVD), atomic layerdeposition (ALD), molecular beam epitaxy (MBE), electroplating,electro-less plating, and the like. Specific embodiments are describedherein with reference to examples of such processes. However, thepresent disclosure and the reference to certain deposition techniquesshould not be limited to those described. For example, in somecircumstances, a description that references CVD may alternatively bedone using PVD, or a description that specifies electroplating mayalternatively be accomplished using electro-less plating. Furthermore,reference to conventional techniques of thin film formation may includegrowing a film in situ. For example, in some embodiments, controlledgrowth of an oxide to a desired thickness can be achieved by exposing asilicon surface to oxygen gas or to moisture in a heated chamber.

Reference throughout the specification to conventional photolithographytechniques, known in the art of semiconductor fabrication for patterningvarious thin films, includes a spin-expose-develop process sequencetypically followed by an etch process. Alternatively or additionally,photoresist can also be used to pattern a hard mask (e.g., a siliconnitride hard mask), which, in turn, can be used to pattern an underlyingfilm.

Reference throughout the specification to conventional etchingtechniques known in the art of semiconductor fabrication for selectiveremoval of polysilicon, silicon nitride, silicon dioxide, metals,photoresist, polyimide, or similar materials includes such processes aswet chemical etching, reactive ion (plasma) etching (RIE), washing, wetcleaning, pre-cleaning, spray cleaning, chemical-mechanicalplanarization (CMP) and the like. Specific embodiments are describedherein with reference to examples of such processes. However, thepresent disclosure and the reference to certain deposition techniquesshould not be limited to those described. In some instances, two suchtechniques may be interchangeable. For example, stripping photoresistmay entail immersing a sample in a wet chemical bath or, alternatively,spraying wet chemicals directly onto the sample.

Specific embodiments are described herein with reference to analogmicrocircuits and FD-SOI transistor devices that have been produced;however, the present disclosure and the reference to certain materials,dimensions, and the details and ordering of processing steps areexemplary and should not be limited to those shown.

In the figures, identical reference numbers identify similar features orelements. The sizes and relative positions of the features in thefigures are not necessarily drawn to scale.

FIGS. 1A and 1B provide general information about SOI transistors,familiar to those skilled in the art of transistor design. FIG. 1A showsa partially-depleted MOS SOI transistor 100 in cross section. Likestandard bulk MOS transistors, the MOS SOI transistor 100 is athree-terminal device in which a voltage applied to a gate 102 causescurrent to flow between a source region 104 and a drain region 106through a channel 108. The gate 102 is separated from the rest of thedevice by a thin capacitive gate oxide layer 110. The MOS SOI transistor100 differs from a bulk MOS transistor in that there exists a buriedoxide (BOX) layer 112 between the channel 108 and a bulk siliconsubstrate 114. A depletion region 116, depleted of charge, that formsbelow the channel 108, between the source and drain regions 104 and 106,is then bounded below by the BOX layer 112. Normally, the presence ofthe BOX layer prevents the substrate voltage from electricallyinfluencing the channel 108. The extent of the depletion region thendepends on the relative dimensions of the various layers, as well assource and drain doping profiles, 117 and 118, respectively, and dopingconcentrations of the source and drain regions. In the case of thepartially-depleted SOI device shown in FIG. 1A, the depletion region 116does not fill all of the material between the source and the drain,wherein an un-depleted portion 119 remains at an undetermined floatingelectric potential. The presence of the un-depleted portion 119 isgenerally undesirable because it is not well controlled, and yet theassociated floating electric potential can electrically influence thechannel and degrade the transistor performance.

An FD-SOI transistor 120 is shown in FIG. 1B in cross section. Like thepartially depleted SOI transistor 100 shown in FIG. 1A, the FD-SOItransistor 120 also has a BOX layer 112. However, the source and drainregions, 124 and 126 respectively, of the FD-SOI device, are shallowerthan the corresponding source and drain regions, 104 and 106,respectively, of the partially depleted SOI transistor 100. As a result,doping profiles 127 and 128 are effectively vertical, and the chargecharacteristics of the channel can be set by the doping concentrationssuch that a fully charge-depleted region 116 forms between the shallowsource and drain regions 124 and 126, bounded below by the BOX layer112, in response to application of a bias voltage to the gate 102.Because all of the material between the source and drain ischarge-depleted, the un-depleted portion 119 shown in FIG. 1A has beeneliminated as a possible cause of transistor degradation.

FIG. 2 shows a generalized example of the architecture of a conventionaldual gate SOI transistor 130 as shown in U.S. Patent Publication No.2010/0264492. Like the conventional MOS and SOI devices, theconventional dual gate SOI transistor 130 has a primary gate 102, asource region 104 and a drain region 106 on either side of a channel108, wherein the primary gate 102 is separated from the channel region108 by a thin primary gate oxide layer 110. In addition, theconventional dual gate SOI transistor 130 includes a secondary gate 132,which is separated from the channel region 108 by a thin, secondary gateoxide layer 134. Both the primary and secondary gates 102 and 132 can bebiased so as to influence current flow in the channel region 108, whichis common to both gates. While the structure of FIG. 2 is known, its useas a capacitor in an operating circuit, such as an amplifier, is notknown and is one embodiment of the invention as explained with respectto FIGS. 4B and 9C.

FIG. 3 shows a conventional reference voltage-based high voltage outputbuffer circuit 140 that uses low voltage devices. The high voltageoutput buffer circuit 140 includes a reference voltage generator stage142 and an output driver stage 144. The output driver stage 144 as shownis a high voltage output driver. The reference voltage generator stage142 includes a voltage divider 146 having an associated referencevoltage V_(REFH0), an operational amplifier (op-amp) 148, a capacitorC1, a transistor T1, and a current source 11. In FIG. 3, the capacitorC1 is a standard, well known parallel plate capacitor. The op-amp 148includes three terminals: an inverting input V⁻, a non-inverting inputV⁺, and an output V_(out). An additional terminal V_(s) is coupled tothe power source V_(DDE). Components internal to the op-amp 148 causethe differential input voltage ΔV_(IN)=V⁺-V⁻ to appear at the outputterminal V_(out), multiplied by an amplification factor, or gain, A,such that V_(out)=AΔV_(IN). The output V_(OUT) of op-amp 148 isconnected to the gate of transistor T1 and its effective voltage isrepresented as V_(BIAS). The op-amp 148 and transistor T1 are connectedin negative feedback to form a unity gain buffer for input referencevoltage V_(REFH0) to produce the output reference voltage V_(REFH) whichwill be used in the output driver 144. The capacitor C1 is used toimprove the stability of negative feedback and is coupled betweenV_(DDE) and V_(BIAS) so that the voltage ΔV across C1 isV_(DDE)-V_(BIAS).

To improve performance of the high voltage output driver stage 144, itis desirable to maximize the value of V_(REFH), and in turn, to maximizeV_(BIAS). Furthermore, it may be desirable to substitute an MOStransistor for the capacitor C1. The effective capacitance of the MOStransistor varies with applied voltage. It can be useful, when designingsuch an MOS device, to evaluate the effective capacitance for differentapplied voltages ΔV by plotting a C-V curve that is determined fromsimulated or actual measurements of the MOS device. Such a C-V curve isshown in FIG. 4. As V_(BIAS) increases, the voltage across C1 decreasesand the effective value of C1 decreases according to the curve 152. Asthe effective value of C1 decreases, the stability of negative feedbackin reference voltage generator stage 142 degrades and hence leads thereference voltage generator circuit towards instability, varyingsignificantly in response to small changes in the voltage across C1. Itis desirable to reduce such bias-dependent behavior of the referencevoltage generator stage 142 by shifting the C-V curve 152 to the left,to produce a desired C-V curve 154 that has an extended flat region 155.According to the C-V curve 154, at around 0V, the capacitance value C1remains substantially constant in response to variations in the voltageacross C1.

The present inventor has realized that, while it is not possible toproduce the desired C-V curve 154 when C1 is a conventional bulk MOStransistor, it is possible to achieve an improved C-V curve when a dualgate SOI transistor of the type shown in FIG. 2 is used. While this dualgate SOI transistor of FIG. 2 is known as a standalone component, theinventors have realized that using it in a circuit of the type shown inFIG. 5 provides a first level of improved behavior of the C-V curve andcircuit operation, as shown in FIG. 8D, explained later.

In addition, the inventors also realized that a significantly improvedC-V curve 154 results when C1 is formed using a dual gate PMOS FD-SOItransistor. One such circuit configuration that incorporates a dual gatePMOS FD-SOI transistor as the capacitor C1 is shown in FIG. 5 as theFD-SOI reference voltage based output driver circuit 160 having amodified reference voltage generator stage 162. Comparing FIG. 5 withFIG. 3, in the modified reference voltage generator stage 162, C1 hasbeen replaced by a dual gate SOI transistor. In one embodiment, the dualgate SOI transistor is a dual gate PMOS FD-SOI transistor 164 indicatedby a dotted circle. A primary gate of the dual gate SOI or dual gatePMOS FD-SOI transistor 164 is coupled to the output of the op-amp 148 atthe voltage V_(BIAS). Source and drain terminals of the dual gate SOI ordual gate PMOS FD-SOI transistor 164 are both coupled to the supplyvoltage V_(DDE), and the secondary gate 132 of the dual gate SOI or dualgate PMOS FD-SOI transistor 164 is grounded.

A portion of the modified reference voltage generator stage 162 shown inFIG. 5 is further detailed in FIG. 6A, in which the dual gate PMOSFD-SOI transistor 164 is represented in physical cross-section with someschematic lines. One alternative embodiment of the cross-section shownin FIG. 6A more clearly shows the capacitive structure of the dual gatePMOS FD-SOI transistor 164. FIG. 6A can be compared with thecross-section of the conventional dual gate SOI transistor 130 shown inFIG. 2. In the dual gate SOI transistor 130 of FIG. 2, the channel 108can be controlled by either a voltage applied to the primary gate 102,or by a voltage applied to the secondary gate 132. When such atransistor is used as a capacitor, the voltage difference between theprimary and secondary gates is applied across both gate dielectrics 110,134.

Turning now to FIG. 6A, parts of the dual gate PMOS FD-SOI transistor164 include the primary gate 102, the source 104, the drain 106, and theBOX 112. The source 104 and drain 106 are doped P-type and the channel165 is doped N-type. Inventive parts of the PMOS FD-SOI transistor 164include a BOX-influenced channel 165, an NWELL region 167 used as asecondary gate, and an NWELL contact 166. In the embodiment shown inFIG. 6A, the function of the secondary gate 132 is carried out by thesubstrate, which is doped to form the NWELL region 167, in oneembodiment. A voltage can be applied to the NWELL region 167 of thesubstrate via the front side NWELL contact 166, which is grounded in theembodiment shown. The secondary gate, NWELL region 167, is spaced apartfrom the channel 165 by a large gate oxide layer, which, in theembodiment shown in FIG. 6A, is the BOX layer 112. An alternative, flipwell embodiment shown in FIG. 6B provides a PWELL 168 as the secondarygate.

FIG. 6C shows schematically the capacitances present in the dual gatePMOS FD-SOI transistors 164 and 163 shown in FIGS. 6A and 6B,respectively. Between V_(BIAS) and ground, there exist at least twocapacitances, C_(gate) across the primary gate oxide layer 110 using thegate 102 and the source/drain/channel as the first and secondelectrodes, respectively, and C_(box) across the BOX layer 112 that usesthe well region as one electrode and the source/drain/channel as theother electrode. The BOX layer 112 provides a much thicker dielectricthan does the gate oxide layer 110. Hence, higher voltage can be appliedacross the BOX layer 112 by biasing the back gate terminal NWELL of thePMOS at a desired voltage level for example, in one configuration shownin FIG. 6A, it is connected to the ground. By connecting the NWELL atground it helps in creating the channel from the back side and hence itincreases the effective value of C_(GATE) capacitance at a lower valueof V_(DDE)-V_(BIAS) at which the channel produced by front gate is weak.Therefore by using the backgate bias voltage the C-V curve 152 ismodified to produce the desired C-V curve 154. In addition to improvingthe C-V behavior of C_(GATE) by connecting the NWELL to ground, the backgate gives an additional capacitance C_(box), connected between V_(DDE)and ground, and hence it works as a decoupling capacitor between supplyand ground.

FIG. 7 shows high level steps in a method 170 of making the dual gatePMOS FD-SOI transistor 164.

At 172, a starting material is provided as a silicon-on-insulator (SOI)semiconductor wafer that includes the BOX layer 112 deposited over aheavily n-doped region, referred to as the NWELL region 167. The BOXlayer 112 has a thickness greater than the front gate so that it cansustain application of up to about ±3.3 V to the NWELL region 167without experiencing a structural breakdown.

At 174, a field effect transistor is formed by conventional methods usedfor SOI wafers to provide a top layer of silicon that covers the BOXlayer 112. The fully depleted (FD) channel 165 can be achieved by makingprofiles of the source and drain regions substantially vertical so theyare perpendicular to the BOX layer 112.

At 176, the front side NWELL contact 166 can be formed to couple aselected voltage to the secondary gate, which is the NWELL region 167.The front side contact can be made of one or more typical contact metalssuch as aluminum, tungsten, gold, and the like, or combinations thereof.

A method of operation in which the dual gate PMOS FD-SOI transistor 164is used as the integrated capacitor C1 entails grounding the NWELLregion 167 acting as the secondary gate, coupling both the source anddrain regions 104 and 106, respectively, to the supply voltage, V_(DDE),and applying a voltage V_(BIAS) to the primary gate 102.

FIGS. 8A-8C show three different options for the capacitor C1 for usein, for example, the reference voltage generator circuits 140 and 160.FIG. 8D compares the simulated device performance of each capacitoroption. With reference to FIG. 8A, C1 is a conventional integrated PWELLcapacitor of the type well known in the art. The PWELL capacitor issimulated with one plate tied to a supply voltage V_(DDE)=3.3 V whilethe other plate is tied to a variable bias voltage V_(BIAS). The supplyvoltage V_(DDE)=3.3 V can be considered a maximum value of V_(BIAS).

Turning now to FIG. 8B, a capacitor option is shown in which C1 isreplaced by a dual gate SOI transistor, CPMOS0, as shown in FIG. 2. Theprimary gate 102 is biased at the voltage V_(BIAS) and the secondarygate 132, source 104, and drain 106 are all tied to the supply voltageat 3.3 V. Thus, the secondary gate 132 is not used as a fourth terminalseparate from the source and drain. This connection provides improvedperformance, as will be shown.

With reference to FIG. 8C, the capacitance C1 is provided by the dualgate PMOS FD-SOI transistor 164 in which the primary gate 102 is biasedat the voltage V_(BIAS), the source 104, and drain 106 are both tied tothe supply voltage at 3.3 V, and the secondary gate 132 is grounded.When the dual gate PMOS FD-SOI transistor 164 is substituted for thecapacitor C1, it provides significantly improved performance for use inthe circuit of FIG. 5.

FIG. 8D presents device simulations of the alternative embodiments forproviding the capacitance C1 shown in FIGS. 8A-8C. A family of C-Vcharacteristic plots 190 comparing simulated performance of thedifferent capacitor options at the device level shows that the FD-SOIreference voltage-based output driver circuit 160 having a modifiedreference voltage generator stage 162 would significantly outperform theconventional output buffer circuit 140 having a reference voltagegenerator stage 142 with a well capacitor. The family of C-Vcharacteristic plots 190 shows capacitance values calculated infemtofarads, at various bias voltages for the three choices of C1described above.

The C-V curve 192 corresponds to the PWELL capacitor C1 of FIG. 8A. Thecapacitance value starts out high, at about 200 fF when V_(BIAS) isunder 1.5 volts, but declines gradually to about 100 fF at V_(BIAS) of3.0 volts and to only 65 fF when V_(BIAS) is 3.3 volts.

The C-V curve 194 corresponds to a dual gate transistor CPMOS0, such asthe one shown in FIG. 2, exhibiting a more abrupt change, however, thetransition occurs at a low voltage. In contrast to the C-V curve 192,the capacitance value drops rapidly for curve 194 associated with theconventional dual gate PMOS transistor CPMOS0, starting to decline atabout 2.5 volts and falling in a near vertical line to reach a lowcapacitance, below 40 fF when V_(BIAS) is about 2.9 volts. As V_(BIAS)reaches its maximum voltage of 3.3 volts, the conventional dual gatePMOS transistor CPMOS0 has a value of only about 35 fF as noted when theline 194 crosses the dotted line 198 at 3.3 volts.

The C-V curve 196 corresponds to the dual gate FDSOI transistor 164described herein, CPMOS1. Over the entire range of V_(BIAS), thetransistor 164 performs better than the standard capacitor or theconventional dual gate transistor, because the capacitance remainsconstant up to the highest value of V_(BIAS), and the capacitance valueat 3.3 V is the highest. The improved device performance indicated bythe simulation curve 196 predicts a commensurate improvement in circuitperformance if the dual gate FDSOI transistor 164 is substituted for thecapacitor C1 in the circuit of FIG. 5. In particular, the C-V curve 196shows a capacitance value of 130 fF at a bias voltage of 3.3V, which istwice the best value measured for the reference voltage generator stage142. Thus, the dual gate PMOS FD-SOI transistor 164, when used as thecapacitor C1 and connected as shown, is expected to operate closer to aconstant, stable, capacitance value, i.e., in the flat region of the C-Vcurve, as opposed to the steep linear region or, worse, the bottom ofthe C-V curve for most of the values of V_(BIAS) from 0 volts to about3.0 volts, which fails to provide a significant capacitance value atapplied bias voltages in the range of about 2.6-3.3 V.

FIG. 9A shows a conventional charge-pumped phase locked loop (CP-PLL)circuit 200. The conventional CP-PLL circuit 200 includes an input stage202, a filter stage 204, and an output stage 206 separated from thefilter stage 204 by a PMOS transistor 208. In one embodiment, the inputstage 202 includes a phase frequency detector and a charge pump, and thefilter stage 204 includes two loop filter capacitors C2 and C3 coupledbetween a supply voltage V_(DD) and the gate of the PMOS transistor 208.The output stage 206 includes a current controlled oscillator (CCO). Theoutput stage 206 is coupled to the input stage 202, forming a feedbackcontrol loop. A de-coupling capacitor C4 is connected between V_(DD) andground.

To improve performance of the conventional CP-PLL circuit 200, it isdesirable to reduce variation of the loop filter capacitors C2 and C3when the conventional CP-PLL circuit 200 is operated at a lower supplyvoltage for example V_(DD)=1.2 V. The present inventor has understoodthat it is possible to achieve the much improved CP-PLL performance byreplacing each of capacitors C2 and C3 with a PMOS FD-SOI transistor164, and also by replacing the de-coupling capacitor C4 with a flip-welldual gate NMOS FD-SOI transistor 210. Such a circuit configuration thatincorporates a dual gate PMOS FD-SOI transistor 164 in place of thecapacitors C2 and C3, and a flip-well dual gate NMOS FD-SOI transistor210 in place of the de-coupling capacitor C4, is shown in FIG. 9B as theFD-SOI charge-pumped phase locked loop (CP-PLL) circuit 212, having amodified filter stage 214. Comparing FIG. 9B with FIG. 9A, in themodified filter stage 214, primary gates of the dual gate PMOS FD-SOItransistors 164 are coupled to the output of the charge pump stage 202at the voltage V_(BIAS) and to the gate of the PMOS transistor 208;source and drain terminals of the dual gate PMOS FD-SOI transistors 164are coupled to the supply voltage V_(DD); and the secondary gates of thedual gate PMOS FD-SOI transistors 164 are grounded. Furthermore, bothprimary and secondary gates of the flip-well dual gate NMOS FD-SOItransistor 210 are coupled to the supply voltage V_(DD), while thesource and drain terminals of the flip-well dual gate NMOS FD-SOItransistor 210 are grounded. A capacitor of the type shown in FIG. 8Bcould also be used for C2-C4 in the circuit of FIG. 9B.

The modified de-coupling capacitor C4 shown in FIG. 9B is furtherdetailed in FIGS. 10A and 10B in which the flip-well dual gate NMOSFD-SOI transistor 210 is represented in cross-section, andschematically, respectively. The cross-sectional view shown in FIG. 10Amore clearly shows the capacitive structure of the flip-well dual gateNMOS FD-SOI transistor 210, in which the source 104 and drain 106 aredoped N-type, and the channel 165 is lightly doped P-type. The flip-welldual gate NMOS FD-SOI transistor 210 shown in FIG. 10A can be compareddirectly with the dual gate PMOS FD-SOI transistor 164 shown in FIG. 6A.For the application described herein, the source and drain terminals ofthe flip-well dual gate NMOS FD-SOI transistor 210 are grounded. In theflip-well dual gate NMOS FD-SOI transistor 210, the channel 165 can becontrolled by either a voltage V_(BIAS) applied to the primary gate 102,or by a voltage applied to the secondary gate 167. In the embodimentshown, the secondary gate is the substrate which is doped to form anNWELL region 167. A supply voltage V_(DD) can be applied to the NWELLregion 167 of the substrate via a front side NWELL contact 166. Thesecondary gate is spaced apart from the channel 165 by the gate oxidewhich, in the embodiment shown, is the same as the BOX layer 112.

In the embodiment shown in FIG. 10A, the NMOS device is formed in anNWELL, instead of the conventional way, which would be in a PWELL. Sucha device is known to those skilled in the art of FD-SOI devices as aflip-well FD-SOI transistor, e.g., CNMOS2FW. An alternative embodimentshown in FIG. 10B provides a PWELL 168 as the secondary gate.

FIG. 10C shows schematically the capacitances present in the flip-welldual gate NMOS FD-SOI transistors 210 and 211 shown in FIGS. 10A and10B, respectively. There exist two parallel capacitances within the dualgate NMOS FD-SOI transistor, C_(gate) across the regular gate oxidelayer 110, and C_(box) across the BOX layer 112. C_(box) thus acts as ade-coupling capacitor between V_(DD) and ground. Hence, the size of thede-coupling capacitor needed can be smaller. In addition, by connectingthe back gate NWELL to VDD, a channel is created from the back side, andhence improves the C-V behavior of C_(gate) at a lower value ofV_(BIAS).

A method of operation in which the flip-well dual gate NMOS FD-SOItransistor 210 is used as the integrated capacitor C4, entails groundingthe source and drain terminals 104 and 106, respectively, coupling theNWELL region 167 of the substrate to the supply voltage, V_(DD), andapplying a bias voltage to the primary gate 102.

FIGS. 11A-11D show four different options for providing a decouplingcapacitance (DCAP) C4 for use in a FD-SOI CP-PLL circuit 212, along witha comparison of the simulated device performance of each option. In FIG.11A, DCAP C4 is an NWELL parallel plate capacitor, having one plate tiedto the variable bias voltage and the other plate grounded.

With reference to FIG. 11B, the capacitance C4 is provided by a dualgate SOI transistor CNMOS0, in which the primary gate is biased at thevoltage V_(BIAS) and the secondary gate, source, and drain are allgrounded.

With reference to FIG. 11C, in a configuration as presented herein, C4is replaced by the flip well dual gate NMOS FD-SOI transistor 210 inwhich the primary gate is biased at the voltage V_(BIAS), the source,and drain are both grounded, and the secondary gate is tied to thesupply voltage at 3.3 V.

With reference to FIG. 11D, in a configuration as presented herein, C4is replaced by a dual gate NMOS FD-SOI transistor 211 in which theprimary gate is biased at the voltage V_(BIAS), the source, and drainare both grounded, and the secondary gate is tied to the supply voltageat 3.3 V. The dual gate NMOS FD-SOI transistor 211 is similar to theflip-well dual gate NMOS FD-SOI transistor 210, except that thesubstrate is P-doped to form a PWELL instead of the NWELL 167.

FIG. 11E shows superior performance of the flip-well dual gate NMOSFD-SOI transistor 210 operated according to the method of operation 220.A family of C-V characteristic plots 230 provides a comparison ofdifferent options for the DCAP in the FD-SOI CP-PLL circuit 212, havinga modified filter stage 214. The family of C-V characteristic plots 230shows capacitance values calculated in femtofarads, at various biasvoltages for the four choices of C4 described above. For example, at themaximum bias voltage of 1.8 V, all of the choices for C4 delivercapacitance values 232 in the range of about 180-200 fF. However, as thebias voltage decreases to as low as 0.2 V as indicated by the dottedline 233, capacitance values indicated by the C-V curves 234 and 236,corresponding to the conventional CNWELL parallel plate capacitor C4 andthe CNMOS0 transistor, respectively, drop to about 125 fF and 35 fF,respectively. Meanwhile, the C-V curves 238 and 239 simulating the NMOSFD-SOI transistor 211, and a flip-well version of the NMOS FD-SOItransistor 210 respectively, used for the capacitor C4, show capacitancevalues are maintained in the range of about 170-180 fF. Thus, the dualgate NMOS FD-SOI transistor 210, operates at a constant, stable,capacitance value, i.e., in the flat region of the C-V curve, as opposedto the steep linear region or, worse, the bottom of the C-V curve, whichfails to provide a significant capacitance value at operating voltagesbelow about 0.6 V. The simulated C-V curves for the various devicesshown in FIGS. 11A-11D thus suggest that the transistor 210 wouldoutperform the other capacitance options when substituted for C4 in thecircuit 212. Furthermore, the simulations shown in FIG. 11E indicatethat the flip-well NMOS FD-SOI transistor 210 significantly outperformsthe NMOS FD-SOI transistor 211 that is formed in a PWELL according tothe standard method.

FIGS. 12A-12E show four different options for operating P-dopedcapacitive devices at a supply voltage V_(DD)=1.2 V, along with acomparison of the simulated device performance of each option. In oneconventional configuration shown in FIG. 12A, a standard parallel platePWELL capacitor has one plate tied to V_(DD) while the other plate istied to a variable bias voltage, V_(BIAS). In another configuration, adual gate transistor device, CPMOS0, has V_(BIAS) applied to the primarygate while the secondary gate, source, and drain are all tied to thesupply voltage at 1.2 V. In third and fourth configurations as describedherein, dual gate PMOS FD-SOI transistors CPMOS1 164 and CPMOS2FWflip-well transistor 163 have a primary gate biased at the voltageV_(BIAS), while the source, and drain are both tied to the supplyvoltage at 1.2 V, and the secondary gate is grounded.

FIG. 12E shows superior performance of the dual gate PMOS FD-SOItransistor 164, operated at the 1.2 V supply voltage according to themethod of operation 180. A family of C-V characteristic plots 240provides a comparison of the four device options shown in FIGS. 12A-12D.The family of C-V characteristic plots 240 shows capacitance valuescalculated in femtofarads, at various bias voltages for the four optionsdescribed above. The C-V curves 241, 242, and 246 for the conventionalCPWELL, CPMOS0, and CPMOS2FW devices, respectively show that thecapacitance values decrease sharply as the variable bias voltageV_(BIAS) exceeds about 0.6 V. In such cases, an additional passive PWELLcapacitor is needed. In contrast, the C-V curve 244 that corresponds tothe dual gate FD-SOI devices CPMOS1 164, still provides capacitances at,for example, a high bias voltage of 0.9 V, exceeding 100 fF, comparedwith about 35 fF for the other devices. Thus, the inventive dual gatePMOS FD-SOI transistors when used with a supply voltage of 1.2 V,provide higher capacitance values for a wider range of bias voltagesthan the other devices provide. In such cases, the passive PWELLcapacitor can be replaced with an active PMOS device. The simulationsshown in FIG. 12E indicate that the flip-well PMOS FD-SOI transistor 163performs only marginally better than the CPMOS0 transistor 242, whilethe performance of the PMOS FD-SOI transistor 163 formed in an NWELLsignificantly outperforms the CPMOS0 bulk transistor.

FIGS. 13A-13D show four different options for operating N-dopedcapacitive devices at a supply voltage V_(DD)=1.2 V, along with acomparison of the performance of each option. In one conventionalconfiguration a standard parallel plate NWELL capacitor has one platetied to V_(BIAS) while the other plate is grounded. In anotherconfiguration, a dual gate SOI transistor device, CNMOS0, has V_(BIAS)applied to the primary gate while the secondary gate, source, and drainare all grounded. In third and fourth configurations as describedherein, dual gate NMOS FD-SOI transistors CNMOS1 and CNMOS2FW have aprimary gate biased at the voltage V_(BIAS), while the source and drainare both grounded, and the secondary gate is tied to the supply voltageat 1.2 V.

FIG. 13E shows superior performance of the dual gate NMOS FD-SOItransistor 210, operated at the 1.2 V supply voltage according to themethod of operation 220. A family of C-V characteristic plots 250provides a comparison of the four device options shown in FIGS. 15A-15D.The family of C-V characteristic plots 250 shows capacitance valuescalculated in femtofarads, at various bias voltages for the four optionsdescribed above. The C-V curves 251 and 252 for the conventional CNWELLand CNMOS0 devices, respectively, show that the capacitance valuesdecrease sharply as the variable bias voltage V_(BIAS) drops below about0.6 V. In contrast, the C-V curves 254 and 256, that correspond to theinventive dual gate FD-SOI devices CNMOS1 211 and CNMOS2FW flip-well210, still provide relatively high capacitances at, for example, a lowbias voltage of 0.4 V, of about 160-180 fF, compared with only about70-145 fF for the conventional devices. Thus, the dual gate NMOS FD-SOItransistors 211 and 210 when used with a supply voltage of 1.2 V,provide higher capacitance values for a wider range of bias voltagesthan the conventional devices provide. The simulations shown in FIG. 13Eindicate that the flip-well NMOS FD-SOI transistor 210 outperforms theNMOS FD-SOI transistor 211 that is formed in a PWELL according to thestandard method.

In addition to improvements in performance and stability of analogcircuit applications that use dual gate FDSOI MOSFET devices in place ofcapacitors, such substitutions have other advantages. For example, whenpassive capacitors can be eliminated from analog circuit designs bysubstituting dual gate FDSOI MOSFET devices, an additional mask is nolonger needed to form the passive capacitors. Thus, the fabricationprocess is simplified, especially for high voltage designs having asupply voltage in the range of about 1.8 V-3.6 V. A simpler processtends to produce higher yields. In addition, the chip real estate neededfor the dual gate FDSOI MOSFET capacitors can be up to 50% smaller thanthat needed for conventional passive capacitors for the same value ofcapacitance needed to stabilize an analog node. Another circuit sizereduction is possible because fewer decoupling capacitors are needed dueto the additional capacitance provided by the BOX layer present in theSOI substrate. As the thickness of the BOX layer shrinks in futuretechnology generations, the associated de-coupling capacitance willincrease and allow further savings in chip real estate.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

It will be appreciated that, although specific embodiments of thepresent disclosure are described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the present disclosure. Accordingly, the present disclosure isnot limited except as by the appended claims.

In the embodiments discussed here the back gate of FD-SOI devices isconnected to V_(DD) for NMOS FD-SOI devices and connected to ground forPMOS FD-SOI devices. However, it is not so limited. The back gatealternatively can be connected to an intermediate voltage, provided thatthe intermediate voltage is sufficient to create the channel from theback side.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A microcircuit, comprising: a voltagesupply; an operational amplifier electrically coupled to the voltagesupply, the operational amplifier having inverting and non-invertinginput terminals and an output terminal; and a dual gate capacitive fieldeffect transistor including: a primary gate coupled to the outputterminal of the operational amplifier; a secondary gate coupled to aground reference voltage; a source region coupled to the voltage supply;and a drain region coupled to the voltage supply, the transistorincluding a doped substrate and a buried oxide layer on the dopedsubstrate, and a decoupling capacitor having a first electrode includingthe source and drain regions and a second electrode including thesecondary gate.
 2. The microcircuit of claim 1 wherein the dopedsubstrate functions as the secondary gate.
 3. The microcircuit of claim1 wherein the primary and secondary gates of the capacitive dual gatefield effect transistor are coupled to different voltages to createdifferent charge characteristics in a channel.
 4. The microcircuit ofclaim 1, further comprising: a current source coupled to the invertinginput terminal; a transistor coupled between the output terminal and thecurrent source; and a voltage divider coupled to the voltage supply andthe non-inverting input terminal; the microcircuit being operable as areference voltage generator.
 5. A microcircuit, comprising: an inputstage that receives an input signal at an input frequency; an outputstage that produces an output signal at an output frequency, the outputsignal coupled to the input stage; and a filter stage coupled betweenthe input stage and the output stage, the filter stage including a pairof capacitors in the form of dual gate transistors, each of the pair ofdual gate transistors including: a primary gate coupled to an output ofthe input stage; a secondary gate coupled to a reference voltage; andsource and drain regions coupled to a voltage supply, the filter stagefurther including a decoupling capacitor in the form of a dual gatetransistor that includes: a primary gate coupled to the voltage supply;a secondary gate coupled to the voltage supply; and source and drainregions coupled to an electrical ground.
 6. The microcircuit of claim 5wherein the filter stage is operable as a loop filter.
 7. Themicrocircuit of claim 5 wherein the capacitors in the filter stage arePMOS dual gate transistors and the decoupling capacitor is an NMOS dualgate transistor.
 8. The microcircuit of claim 5 wherein one or more ofthe dual gate transistors is a fully-depleted SOI dual gate transistor.9. The microcircuit of claim 5 wherein the input stage includes a phasefrequency detector and a charge pump; the output stage includes a PMOSfield effect transistor and a current controlled oscillator; and themicrocircuit is operable as a charge pumped phase locked loop circuit.10. A device, comprising: a silicon-on-insulator (SOI) substrateincluding a doped well having a first conductivity type and a buriedoxide layer on the doped well; a capacitive dual gate field effecttransistor (FET) in an active layer of the SOI substrate, the capacitivedual gate FET including a channel region and source and drain regions,the source and drain regions having a second conductivity type andhaving respective substantially vertical profiles in abutting contactwith the channel region and oriented perpendicular to the buried oxidelayer, the source and drain regions being commonly coupled to a firstvoltage; and a front side contact to the doped well having the firstconductivity type, the front side contact and the doped well beingcoupled to an electrical ground, wherein the source region, drain regionand channel region operatively form a first electrode of a decouplingcapacitor, and the doped well operatively forms a second electrode ofthe decoupling capacitor.
 11. The device of claim 10 wherein the dopedwell is an N-well and the FET is a p-type device.
 12. The device ofclaim 10 wherein the doped well is a P-well and the FET is an n-typedevice.
 13. The device of claim 10, further comprising: an interconnectthat couples the source and drain regions to a power supply, the powersupply being configured to supply the first voltage.
 14. The device ofclaim 10, further comprising: a primary gate oxide layer that issubstantially thinner than the buried oxide layer; and a primary gateadjacent to the primary gate oxide layer, the primary gate configured toapply a bias voltage across the primary gate oxide layer.
 15. The deviceof claim 10, wherein the buried oxide layer is configured as a secondcapacitive dielectric.
 16. The device of claim 15 wherein the doped wellis configured as a back gate for applying a bias across the secondcapacitive dielectric.
 17. The microcircuit of claim 5, wherein thereference voltage is the electrical ground.
 18. The microcircuit ofclaim 1 wherein the doped substrate includes an N-well and the dual gatecapacitive field effect transistor is a p-type device.
 19. Themicrocircuit of claim 1 wherein the doped substrate includes a P-welland the dual gate capacitive field effect transistor is an n-typedevice.
 20. The microcircuit of claim 1, the dual gate capacitive fieldeffect transistor further including a primary gate oxide layer betweenthe buried oxide layer and the primary gate.